1. Field of the Invention
The invention relates to a circuit for a video recorder comprising a modulator for modulating a chrominance signal from a first onto a second carrier wave by means of a third carrier wave, a first phase-locked loop in which a first phase comparator circuit controls a first voltage-controlled oscillator in conformity with the phase comparison between the colour-burst signal, and an oscillator signal generated in a reference oscillator. The first voltage-controlled oscillator is coupled to the input of a first mixer for supplying the third carrier wave and is coupled to a second phase-locked loop comprising a second voltage-controlled oscillator and a second phase-comparator circuit for synchronising the frequency-divided oscillator signal generated by the second voltage-controlled oscillator with the reinserted line-synchronising pulses.
2. Prior Art
In recent video recorder systems, the band-limited luminance signal extracted from the composite colour signal is frequency-modulated and transposed to a higher frequency band of, for example, from 3 to 5 MHz during recording. The other signal component of the composite colour signal, the chrominance signal modulated from a chrominance carrier having a frequency of, for example, 4.43 MHz, onto a carrier of lower frequency, for example 627 kHz. During playback the chrominance signal is reconverted to the higher frequency band, which is effected by means of the above circuit arrangement which is known from the Service Manual of the Philips video recorder VR 6460. In said circuit arrangement this modulation is effected by means of a third carrier wave having a frequency of, for example 5.06 MHz, which in a first mixer is derived from the oscillator signals supplied by the first voltage-controlled (crystal) oscillator and the second voltage-controlled (RC) oscillator.
The oscillator signal generated by the second voltage-controlled oscillator is synchronised with the line synchronising pulses extracted from the luminance signal in the second phase-locked loop. The second phase-locked loop eliminates velocity variations of the heads, which give rise to frequency variations.
The frequency and phase of the first voltage-controlled oscillator are determined by the phase comparison between the first oscillator signal from a reference oscillator and the colour burst signal. Said first phase-locked loop should comprise a voltage-controlled cyrstal oscillator in order to avoid locking-in to a sideband of the burst frequency, which may happen with, for example, RC oscillators which have a larger tuning range. Such a sideband frequency is equal to the burst frequency of 4.43 MHz .+-.n.15,625 kHz, 15,625 kHz being the line frequency and n being an integer larger than or equal to 1. Since a crystal oscillator has a smaller tuning range, phase variations of the burst signal are eliminated slowly. Moreover, if this circuit arrangement is integrated, two external crystals must be connected because the first voltage-controlled oscillator and the reference oscillator are each controlled by a separate crystal.